Semiconductor device and manufacturing method therefor

ABSTRACT

A semiconductor device includes a buried impurity layer (3) formed at a predetermined depth from a main surface of a semiconductor substrate (1) by utilizing ion injection of a conductivity type determining element, and a gettering layer (2) formed in a position adjacent to and not shallower than the buried impurity layer (3) by utilizing ion injection of an element other than a conductivity type determining element.

This application is a continuation of application Ser. No. 07/902,424filed Jun. 24, 1992, abandoned which is a continuation-in-partapplication of Ser. No. 07/660,824 filed Feb. 26, 1991 abandoned.

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

This invention relates to a semiconductor device and a manufacturingmethod therefor. More particularly, the invention relates toimprovements in a semiconductor device and a manufacturing methodtherefor, the device including a buried impurity layer formed in asemiconductor substrate by using ion injection.

DESCRIPTION OF THE BACKGROUND ART

It is known that, generally, a buried impurity layer is provided for anintegrated circuit including a plurality of MOS transistors in order toprevent software errors due to alpha particles and to prevent latchups.It is also known that a buried impurity layer is provided for a bipolartransistor to act as a floating collector.

As shown in FIG. 9A which is a sectional view, a buried impurity layer3a, usually, is formed by diffusing an element that determines theconductivity type, such as boron, phosphorus or arsenic, on a mainsurface of a semiconductor substrate 1, and superposing an epitaxiallayer 1a in a thickness of several micrometers on the impurity layer 3a.The epitaxial layer 1a is covered with an insulating film 4 andisolation regions 5. A semiconductor device such as a MOS transistor(not shown) is formed on the epitaxial layer 1a in an area surrounded bythe isolation regions 5. However, it is time-consuming and costly toform the buried impurity layer 3a by diffusion and to cause growth ofthe epitaxial layer 1a.

Thus, as shown in FIG. 9B, attempts have been made in recent years toutilize ion injection in forming a buried impurity layer in a short timeand at a relatively low cost. More particularly, a buried impurity layer3 is formed by ion injection, with a high energy in the range of severalhundred keV to several MeV, of an element that determines theconductivity type, through an insulating film 4 into positions of asemiconductor substrate 1 at a depth of several micrometers. Then thesubstrate 1 is heat-treated in order to activate the buried impuritylayer 3 and to eliminate primary crystal defects due to the ioninjection.

During the heat treatment, the disappearance of the primary crystaldefects in the buried impurity layer 3 due to the ion injectionprogresses inwardly from top and bottom of the impurity layer 3.However, secondary defects such as dislocations and stacking faults tendto remain in inner positions of the impurity layer 3. In the regionsupwardly of the buried impurity layer 3 where the ions have passed, theprimary defects such as vacancies tend to remain which retard recoveryof crystallinity. Such residual defects can be a cause of increases inthe leak current of the substrate.

Referring to FIG. 10, an example of methods for measuring the leakcurrent of a semiconductor substrate 1 including a buried impurity layer3 is illustrated. In FIG. 10, a p⁻ substrate 1 includes a buried p⁺impurity layer 3. An n⁺ impurity region 7 is formed on an upper surfaceof the p⁻ substrate 1. The n⁺ impurity region 7 is connected through anammeter 8 to a variable positive voltage source 9. The substrate 1 has agrounded bottom surface. In this way, the leak current may be measuredby applying a reverse bias voltage to the semiconductor substrate 1.

Referring to FIG. 11A, an example of leak current of a substratemeasured by the method of FIG. 10 is shown. In FIG. 11A, the substrateis injected with boron ions with an accelerating energy of 1.5 MeV in1×10¹⁴ ions/cm², and thereafter annealed in a nitrogenous atmosphere at1000° C. for one hour. The horizontal axis represents the reverse biasvoltage (V), and the vertical axis the leak current (A). It will be seenthat the leak current of the substrate increases markedly at the reversebias voltage of about 3.5V and above, and that this substrate isunavailable for practical purposes. It is believed that this increase inthe leak current is caused by the residual crystal defects due to theion injection.

FIG. 11B shows, for purposes of comparison, a leak current of asemiconductor substrate including a buried impurity layer formed bydiffusing boron and an epitaxial layer superposed thereon. In FIG. 11B,the leak current shows little increase at the reverse bias voltage up toabout 17V since the semiconductor substrate includes no lattice defectsdue to ion injection.

As noted above, a buried impurity layer may be formed in a semiconductorsubstrate in a short time and at low cost by utilizing high energy ioninjection. However, a substrates including a buried impurity layerformed in this way is not fit for practical use because of the greatleak current.

SUMMARY OF THE INVENTION

An object of this invention is to provide a semiconductor deviceincluding a buried impurity layer which may be formed in a short timeand at low cost.

A semiconductor device, according to one aspect of this invention,includes a buried impurity layer formed at a predetermined depth from amain surface of a semiconductor substrate by utilizing ion injection ofa conductivity type determining element, and a gettering layer formed ina position adjacent to and not shallower than the buried impurity layerby utilizing ion injection of an element other than a conductivity typedetermining element.

According to another aspect of this invention, a method of manufacturinga semiconductor device comprises the steps of forming a gettering layerby ion-injecting an element other than a conductivity type determiningelement with a high energy to a predetermined depth from a main surfaceof a semiconductor substrate, and heat-treating the semiconductorsubstrate; and forming a buried impurity layer by ion-injecting aconductivity type determining element with a high energy to a positionnot deeper than the gettering layer, and heat-treating the semiconductorsubstrate.

In the semiconductor device according to this invention, a getteringlayer is formed in a position adjacent to and not shallower than theburied impurity layer formed by utilizing ion injection. Consequently,the crystal defects due to the ion injection are absorbed into thegettering layer. Thus, a semiconductor device is provided which has areduced leak current though it includes the buried impurity layer formedby utilizing ion injection.

In the method of manufacturing a semiconductor device according to thisinvention, a buried impurity layer and a gettering layer are both formedby utilizing ion injection. This method, therefore, enables asemiconductor device including a buried impurity layer and having areduced leak current to be manufactured in a short time and at low cost.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A through 1C are sectional views illustrating a method ofmanufacturing a semiconductor device in one embodiment of thisinvention.

FIGS. 2A through 2C are graphs showing leak currents of semiconductorsubstrates including buried impurity layers and gettering layers formedunder various ion-injecting conditions.

FIG. 3 is a graph showing a relationship between energy and depth ofinjecting oxygen ions into a silicon substrate.

FIG. 4 is a graph showing a leak current of a substrate including agettering layer and a buried impurity layer formed by utilizinginjection of carbon and boron ions,

FIG. 5 is a graph showing a relationship between energy and depth ofinjecting carbon ions into a silicon substrate,

FIGS. 6A and 6B are sectional views showing other embodiments of thisinvention.

FIGS. 7 is a block diagram of a DRAM device including memory cellsformed by utilizing this invention.

FIG. 8 is a sectional view of a memory cell formed by utilizing thisinvention.

FIG. 9A is a sectional view of a semiconductor substrate including aburied impurity layer according to the prior art, which is formed byutilizing diffusion of impurities and epitaxial growth.

FIG. 9B is a sectional view of a semiconductor substrate including aburied impurity layer according to the prior art, which is formed byutilizing ion injection.

FIG. 10 is a conceptual view illustrating a method of measuring leakcurrent of a semiconductor substrate.

FIG. 11A and 11B are graphs showing leak currents of semiconductorsubstrates according to the prior art as shown in FIGS. 9A and 9B,respectively.

FIGS. 12A and 12B are graphs showing leak currents of semiconductorsubstrates according to the prior art including gettering layers formedby utilizing ion injection.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

It is known that gettering technique may be used effectively in order toachieve a high yield in manufacture of LSI devices. Gettering techniqueis intended for introducing crystal defects accompanying stress fields,such as dislocations and precipitated particles, and absorbing into thegettering region and removing harmful heavy metal impurities, pointdefects and the like from regions for forming semiconductor devices suchas MOS transistors.

Recently, Wong et al stated in Applied Physics Letter, 52(12), Mar. 21,1988, pp. 1023-1025 that a gettering layer could be formed byion-injecting oxygen element or carbon element with a high acceleratingenergy of several hundred keV to several MeV into positions severalmicrometers deep inside a silicon substrate. A gettering layer formed byutilizing ion injection as above lies near a substrate surface forforming semiconductor devices such as MOS transistors, and hence isexpected to produce a powerful gettering effect.

However, as shown in FIGS. 12A and 12B, semiconductor substratesincluding gettering layers formed by utilizing ion injection tend toshow large leak currents. In FIG. 12A, a gettering layer is formed byheat-treating a semiconductor substrate into which oxygen has beenion-injected in 1×10¹⁵ ions/cm² with an accelerating energy of 2.4 MeV.This substrate shows the leak current increasing at a reverse biasvoltage of about 5V and above. In FIG. 12B, a gettering layer is formedby heat-treating a semiconductor substrate into which carbon has beenion-injected in 1×10¹⁵ ions/cm² with an accelerating energy of 2.0 MeV.This substrate shows the leak current greatly increasing at a reversebias voltage of about 2V and above.

The reason for the large leak current of a semiconductor substrateincluding a gettering layer formed by utilizing ion injection isbelieved to lie in that the reverse bias voltage causes a depletionlayer extending in the direction of depth of the semiconductor substrateto reach crystal defects such as dislocations present in the getteringlayer at shallow positions of several micrometers down, whereby thesecrystal defects cause the increase in the leak current. Thus, agettering layer formed by utilizing ion injection is expected, underordinary circumstances, to further increase the leak current in thesemiconductor substrate including a buried impurity layer formed byutilizing ion injection. In spite of such a situation, the presentinventors have undertaken to form not only a buried impurity layer butalso a gettering layer in the semiconductor substrate by utilizing ioninjection.

A method of manufacturing a semiconductor device in one embodiment ofthis invention will be described with reference to sectional views shownin FIGS. 1A through 1C.

Referring to FIG. 1A first, an oxide film 4 having a thickness ofseveral hundred Å is formed on a surface of a silicon semiconductorsubstrate 1. Then, as indicated by arrows, oxygen is ion-injected in1×10¹⁴ to 1×10¹⁵ ions/cm² with an accelerating energy of several hundredkeV to several MeV to depths of several micrometers into thesubstrate 1. As a result, an oxygen-injected layer 2 is formed. Asuitable range of thickness for this layer is 0.5-5 μm.

Referring to FIG. 1B, isolation oxide layers 5 are formed by selectivethermal oxidation. At this time, secondary crystal defects are formed inthe oxygen-injected layer 2, particularly in central portions thereof,to act as sinks for gettering.

Referring to FIG. 1C, as indicated by arrows, a conductivity typedetermining element such as boron, phosphorus or arsenic is ion-injectedwith a high energy in 1×10¹³ to 1×10¹⁶ ions/cm² to positions not deeperthan the gettering layer 2. As a result, a buried impurity layer 3 isformed a suitable range of thickness for which is 1-5 μm. The buriedimpurity layer 3 is activated through an annealing treatment carried outin a nitrogenous atmosphere at 1000° C. for one hour. The primarydefects due to the ion injection are expected to be absorbed into thegettering layer 2 at this time.

Referring to FIG. 2A, an example of leak current of a semiconductorsubstrate having a construction as in FIG. 1C is shown. The horizontalaxis represents the reverse bias voltage (V), and the vertical axis theleak current (A). In FIG. 2A, the substrate is injected with oxygen ionswith an energy of 2.0 MeV in 1×10¹⁵ ions/cm² and with boron ions with anenergy of 1.5 MeV in 1×10¹⁴ ions/cm².

As seen from FIG. 2A, the leak current of the substrate increases littleat the reverse bias voltage up to about 17V. Thus, the semiconductorsubstrate shown in FIG. 2A, though it includes a buried impurity layerformed by utilizing ion injection, produces a much less leak currentthan the known semiconductor substrate shown in FIG. 11A. Compared withthe substrate shown in FIG. 12A which includes only a gettering layer,the semiconductor substrate shown in FIG. 2A has a greatly reduced leakcurrent particularly at the reverse bias voltage of about 5V and above.

This unexpected reduction of the leak current may be considered to havethe following two reasons (1) and (2):

(1) The primary defects introduced at the time of born ion injection areabsorbed into the gettering layer during the annealing treatment.Consequently, no crystal defects are present at least in positionsshallower than the top of the buried impurity layer.

(2) The buried impurity layer 3 including no crystal defects is formedin a position shallower than the middle portion of the gettering layerhaving a high concentration of secondary defects. Thus, the depletionlayer is restrained from reaching the secondary defects in the middleportion of the gettering layer.

The gettering layer 2 as shown in FIG. 1C does not disappear through thesubsequent heat treatment. Thus, also in the process of forming asemiconductor device such as a MOS transistor on the surface of thesemiconductor substrate 1, the gettering layer 2 produces the getteringeffect for removing harmful heavy metal impurities from adjacent thesubstrate surface. That is, the leak current flowing through thesubstrate may be checked effectively during operation of such asemiconductor device.

Further, the semiconductor substrate of FIG. 2A shows a leak currentsimilar to that of the substrate of FIG. 11B including the buriedimpurity layer 3a formed without utilizing ion injection. However, asemiconductor substrate including a buried impurity layer formed byutilizing diffusion of impurities and growth of an epitaxial layer showsa relatively strong leak current at certain locations. This isconsidered due to the fact that different locations of the substratehave different concentrations of the crystal defects introduced duringthe epitaxial growth and different undesirable concentrations ofimpurities. However, the semiconductor substrate as shown in FIG. 1Cinvolves hardly any variations in the leak current occurring atdifferent locations. The reason is believed to lie in that the getteringlayer 2 uniforms the entire substrate by absorbing crystal defects andundesirable impurities present locally.

FIGS. 2B and 2C resemble FIG. 2A, but in FIGS. 2B and 2C oxygen ision-injected with energies of 2.2 MeV and 2.4 MeV, respectively. Thegettering layer in FIG. 2B is formed at a greater depth than thegettering layer in FIG. 2A. The gettering layer in FIG. 2C is formed ata still greater depth than the gettering layer in FIG. 2B. It will beseen that the gettering layer 2 demonstrates a sufficient getteringeffect despite certain variations in its depth, and that the leakcurrent of the substrate is sufficiently restrained as long as thegettering layer 2 is formed in a position not shallower than the buriedimpurity layer 3.

The relationship between injection energy and injection depth of ionsmay readily be determined experimentally. For example, FIG. 3 shows arelationship between accelerating energy and injection depth in theinjection of oxygen ions into a silicon substrate. The horizontal axisrepresents the accelerating energy of ions, and the vertical axis thedepth of oxygen ions injected into the silicon substrate.

Referring to FIG. 4, this graph shows another example of leak current ofa semiconductor substrate as shown in FIG. 1C. While FIG. 4 resemblesFIG. 2A, in FIG. 4 carbon instead of oxygen is ion-injected with anenergy of 1.6 MeV in 1×10¹⁵ ions/cm². It will be seen that the leakcurrent of the substrate is restrained also by the gettering layer 2formed by utilizing carbon ion injection. As seen from FIG. 5 ascompared with FIG. 3, carbon is ion-injected with a lower acceleratingenergy than oxygen because carbon can be ion-injected deeper than oxygenwith the same accelerating energy.

Further, the gettering layer 2 may be formed by utilizing ion injectionof fluorine, chlorine or nitrogen. However, ion injection of an elementthat determines the conductivity type is not suited for formation of thegettering layer 2. The reasons are that the secondary crystal defectsresulting from the ion injection of a conductivity type determiningelement have a weak gettering function, and that a conductivity typedetermining element, from the viewpoint of its electric function, mustnot be ion-injected in a very high concentration.

In the embodiment of FIGS. 1A through 1C, the buried impurity layer 3 isformed after the isolation oxide layers 5 are formed. However, as shownin FIG. 6A, not only the ion injection for forming the gettering layer 2but also the ion injection for forming the buried impurity layer 3 maybe carried out before formation of the isolation oxide layers 5.Further, as shown in FIG. 6B, both the gettering layer 2 and buriedimpurity layer 3 may be formed by utilizing ion injection afterformation of the isolation oxide layers 5.

It will be understood that, depending on the type of semiconductordevice formed on the semiconductor substrate, the buried impurity layer3 and gettering layer 2 may be formed at desired depths by adjusting theion injecting energy, and quantities of ion injection may also beadjusted.

It will be understood further that, while the isolation oxide layers areused in the foregoing embodiment, trench isolation, planar isolation andother isolation modes are also available.

An example will be described hereinafter wherein a semiconductorsubstrate including a buried impurity layer and a gettering layer formedaccording to this invention is utilized for memory cells in a DRAM(random access memory) device.

Referring to FIG. 7, an ordinary DRAM construction is shown in blockdiagram. A DRAM 50 includes a memory cell array 51 for storinginformation data signals, a row and column address buffer 52 forreceiving from outside address signals for selecting memory cells eachto constituting a unit storage circuit, a row decoder 53 and a columndecoder 54 for designating memory cells by decoding the address signals,a sense refresh amplifier 55 for amplifying and reading signals storedin the designated memory cells, a data input buffer 56 and a data outputbuffer 57 for inputting and outputting data, and a clock generator 58for generating a clock signal.

Referring to FIG. 8 which is a sectional view, there is shown a stackedtype cell included in the memory cell array 51 shown in FIG. 7. In thememory cell of FIG. 8, the semiconductor substrate 1 including theburied impurity layer 3 and gettering layer 2 according to thisinvention is used. The memory cell formed on the substrate 1 includes asource/drain region 7a, a word line 17, a storage node 10, a capacitorinsulating film 11, a cell plate 12, an interlayer insulating film 16and a bit line 14. With this memory cell, the buried impurity layer 3absorbs carriers generated by alpha particles, thereby effectivelypreventing malfunctioning of the DRAM device.

According to this invention, as described above, a semiconductor deviceis provided which includes a buried impurity layer and a gettering layerformed in a short time and at low cost by utilizing ion injection, andwhich involves a reduced leak current.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

What is claimed is:
 1. A semiconductor device, comprising;a localizedburied impurity layer comprising an ion-injected conductivity typedetermining element located at a predetermined depth from a main surfaceof a semiconductor substrate; and a localized gettering layer comprisingan ion-injected element selected from the group consisting of oxygen,carbon, fluorine, chlorine and nitrogen with a dose in the range of1×10¹⁴ to 1×10¹⁵ ions/cm², located at a predetermined depth from a mainsurface of a semiconductor substrate; wherein the localized buriedimpurity layer is formed entirely within the semiconductor substrate ata shallower depth from the main surface of the semiconductor substratethan the localized gettering layer and the localized buried impuritylayer overlaps to a predetermined amount the localized gettering layer.2. A semiconductor device, manufactured by a process comprising thesteps of:forming a conductive localized gettering layer in a thicknessof 0.5-5.0 μm in a semiconductor substrate by ion-injecting a firstelement selected from the group consisting of oxygen, carbon, fluorine,chlorine and nitrogen with a dose in the range of 1×10¹⁴ to 1×10¹⁵ions/cm², with a first high energy to a predetermined depth determinedfrom a main surface of said semiconductor substrate; forming a buriedimpurity layer entirely within and under a main surface of thesemiconductor substrate in a thickness of 1-5 μm by ion-injecting asecond element, selected to be a conductivity type determining element,with a second high energy, to a position not deeper than said getteringlayer; and providing thereafter a heat treatment to said semiconductorsubstrate.
 3. A semiconductor device according to claim 1, wherein:saidsecond element is selected from a group consisting of boron, phosphorusand arsenic, and is ion-injected to a density in the range 1×10¹³ to1×10¹⁶ ions/cm².
 4. A semiconductor device according to claim 1,wherein:said second element is selected from a group consisting ofboron, phosphorus and arsenic, and is ion-injected to a density in therange 1×10¹³ to 1×10¹⁶ ions/cm².
 5. A semiconductor device according toclaim 3, wherein:said second element is ion-injected to a depth notgreater than the depth to which said first element is ion-injected.
 6. Asemiconductor device according to claim 4, wherein:said second elementis ion-injected to a depth not greater than the depth to which saidfirst element is ion-injected.
 7. A semiconductor device according toclaim 2, wherein:said heat treatment step comprises the step ofproviding a nitrogenous atmosphere in which the semiconductor device isheated to be at a temperature of 1000° C. for one hour to therebyactivate the buried impurity layer and generate secondary defects thatserve as gettering sites in the gettering layer.
 8. A semiconductordevice according to claim 6, wherein:said heat treatment step comprisesthe step of providing a nitrogenous atmosphere heated to be at atemperature of 1000° for one hour to thereby activate the buriedimpurity layer and generate secondary defects that serve as getteringsites in the gettering layer.
 9. A semiconductor device, comprising:alocalized buried impurity layer, in a thickness of 1-5 μm, comprising anion-injected conductivity type determining element, and having an uppersurface located at a predetermined depth from a main surface of asemiconductor substrate; and a localized conductive gettering layer,having a thickness in the range 0.5-5.0 μm, comprising an ion-injectedelement selected from the group consisting of oxygen, carbon, fluorine,chlorine and nitrogen with a dose in the range of 1×10¹⁴ to 1×10¹³ions/cm², located in a position adjacent to and not shallower than saidburied impurity layer.
 10. A semiconductor device, comprising:amonolithic substrate having a main surface; source/drain regions of afield effect device formed spaced apart on the main surface of thesubstrate; a conductive gettering layer, having a thickness in the rangeof 0.5-5.0 μm, formed in said substrate, and having an upper surface ata first depth from said main surface of the substrate wherein saidconductive gettering layer comprises an ion-injected element selectedfrom the group consisting of oxygen, carbon, fluorine, chlorine andnitrogen with a dose in the range of 1×10¹⁴ to 1×10¹⁵ ions/cm² ; and aburied impurity layer, having a thickness in the range of 1-5 μm, formedin said substrate and having an upper surface at a second depth fromsaid main surface of the substrate, said second depth being less thansaid first depth.